# std::find, std::find_if, std::find_if_not

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class InputIt, class T > InputIt find( InputIt first, InputIt last, const T& value ); |  | (constexpr since C++20) (until C++26)
template< class InputIt, class T = typename std::iterator_traits <InputIt>::value_type > constexpr InputIt find( InputIt first, InputIt last, const T& value ); |  | (since C++26)
| (2) |
template< class ExecutionPolicy, class ForwardIt, class T > ForwardIt find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value ); |  | (since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type > ForwardIt find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value ); |  | (since C++26)
template< class InputIt, class UnaryPred > InputIt find_if( InputIt first, InputIt last, UnaryPred p ); | (3) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred > ForwardIt find_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPred p ); | (4) | (since C++17)
template< class InputIt, class UnaryPred > InputIt find_if_not( InputIt first, InputIt last, UnaryPred q ); | (5) | (since C++11) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred > ForwardIt find_if_not( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPred q ); | (6) | (since C++17)

Returns an iterator to the first element in the range `[`first`,` last`)` that satisfies specific criteria (or last if there is no such iterator).

1) `find` searches for an element equal to value (using `operator==`).
3) `find_if` searches for an element for which predicate p returns true.
5) `find_if_not` searches for an element for which predicate q returns false.
2,4,6) Same as (1,3,5), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### Parameters

first, last | - | the range of elements to examine
value | - | value to compare the elements to
policy | - | the execution policy to use
p | - | unary predicate which returns ​true for the required element. The expression p(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of VT&is not allowed, nor is VT unless for `VT` a move is equivalent to a copy(since C++11). ​
q | - | unary predicate which returns ​false for the required element. The expression q(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of VT&is not allowed, nor is VT unless for `VT` a move is equivalent to a copy(since C++11). ​
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- `UnaryPredicate` must meet the requirements of Predicate.

### Return value

The first iterator it in the range `[`first`,` last`)` satisfying the following condition or last if there is no such iterator:

1,2) *it == value is true.
3,4) p(*it) is true.
5,6) q(*it) is false.

### Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) At most \(\scriptsize N\)N comparisons with value using `operator==`.
3,4) At most \(\scriptsize N\)N applications of the predicate p.
5,6) At most \(\scriptsize N\)N applications of the predicate q.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

find
    template<class InputIt, class T = typename std::iterator_traits<InputIt>::value_type>
    constexpr InputIt find(InputIt first, InputIt last, const T& value)
    {
        for (; first != last; ++first)
            if (*first == value)
                return first;

        return last;
    }
find_if
    template<class InputIt, class UnaryPred>
    constexpr InputIt find_if(InputIt first, InputIt last, UnaryPred p)
    {
        for (; first != last; ++first)
            if (p(*first))
                return first;

        return last;
    }
find_if_not
    template<class InputIt, class UnaryPred>
    constexpr InputIt find_if_not(InputIt first, InputIt last, UnaryPred q)
    {
        for (; first != last; ++first)
            if (!q(*first))
                return first;

        return last;
    }

### Notes

If C++11 is not available, an equivalent to `std::find_if_not` is to use `std::find_if` with the negated predicate.

    template<class InputIt, class UnaryPred>
    InputIt find_if_not(InputIt first, InputIt last, UnaryPred q)
    {
        return std::find_if(first, last, std::not1(q));
    }

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | List-initialization for algorithms (1,2)

### Example

The following example finds numbers in given sequences.

Run this code

    #include <algorithm>
    #include <array>
    #include <cassert>
    #include <complex>
    #include <initializer_list>
    #include <iostream>
    #include <vector>

    bool is_even(int i)
    {
        return i % 2 == 0;
    }

    void example_contains()
    {
        const auto haystack = {1, 2, 3, 4};

        for (const int needle : {3, 5})
            if (std::find(haystack.begin(), haystack.end(), needle) == haystack.end())
                std::cout << "haystack does not contain " << needle << '\n';
            else
                std::cout << "haystack contains " << needle << '\n';
    }

    void example_predicate()
    {
        for (const auto& haystack : {std::array{3, 1, 4}, {1, 3, 5}})
        {
            const auto it = std::find_if(haystack.begin(), haystack.end(), is_even);
            if (it != haystack.end())
                std::cout << "haystack contains an even number " << *it << '\n';
            else
                std::cout << "haystack does not contain even numbers\n";
        }
    }

    void example_list_init()
    {
        std::vector<std::complex<double>> haystack{{4.0, 2.0}};
    #ifdef __cpp_lib_algorithm_default_value_type
        // T gets deduced making list-initialization possible
        const auto it = std::find(haystack.begin(), haystack.end(), {4.0, 2.0});
    #else
        const auto it = std::find(haystack.begin(), haystack.end(), std::complex{4.0, 2.0});
    #endif
        assert(it == haystack.begin());
    }

    int main()
    {
        example_contains();
        example_predicate();
        example_list_init();
    }

Output:

    haystack contains 3
    haystack does not contain 5
    haystack contains an even number 4
    haystack does not contain even numbers

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 283 | C++98 | `T` was required to be EqualityComparable, but the value type of `InputIt` might not be `T` | removed the requirement

### See also

adjacent_find | finds the first two adjacent items that are equal (or satisfy a given predicate) (function template)
find_end | finds the last sequence of elements in a certain range (function template)
find_first_of | searches for any one of a set of elements (function template)
mismatch | finds the first position where two ranges differ (function template)
search | searches for the first occurrence of a range of elements (function template)
ranges::findranges::find_ifranges::find_if_not (C++20)(C++20)(C++20) | finds the first element satisfying specific criteria (algorithm function object)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/find&oldid=178082"